This invention relates to electrodes, and more particularly to disposable medical electrodes of the type employed in transcutaneous monitoring of biological or physiological electrical potentials associated with muscular activity.
Electrodes of the foregoing type are utilized in a number of applications for variety of purposes. The monitoring of physiological electrical potentials to detect muscular activity of the heart muscle is generally well established, such apparatus being referred to in the art as an electrocardiograph (ECG) apparatus. The resulting traces or electrocardiogram obtained by this apparatus provide a diagnostic tool for detecting heart disease and/or defects. Such monitoring of physiological electrical potentials may of course be employed in a number of other applications. However, the electrode of the present invention will be described herein with particular reference to its use in connection with ECG apparatus.
Such ECG traces or electrocardiograms may be desired in a number of different situations. For example, a simple ECG test to obtain a single tracing for diagnostic purposes may be carried out in a few minutes in a physician's office. Hence, electrodes utilized for such testing may readily be of a relatively simple disposable variety, since they are only in service for a very short time.
On the other hand, longer term monitoring applications require the electrodes to remain in place on the patient's skin for considerably longer periods of time. For example, in stress testing, the heart activity of the patient is monitored over a relatively longer period of time while the patient exercises, for example upon a treadmill or the like. Such testing may include monitoring of the heart activity during the exercise, as well as continued monitoring during a rest period thereafter to monitor the return of the heart to a normal or unstressed condition.
Similarly, electrodes monitoring heart activity during surgery may be required to remain in place and operational for a period of several hours duration. In similar fashion, patients hospitalied in intensive care or other specialized care units may require around-the-clock monitoring. Hence, electrodes utilized for the ECG monitoring over such extended periods must remain in service for many hours and sometimes over a period of several days.
In most of the foregoing applications, two important and competing considerations must be addressed in designing a suitable electrode for ECG monitoring. Initially, it is desirable, especially during simple, short term ECG testing, to obtain a useful trace from the electrode as soon as possible after placing the electrode on the skin. Hence, initially placing and thereafter maintaining the electrode in intimate contact with the skin is an important consideration in rapidly obtaining a usable trace, as well as in maintaining the quality of the trace throughout the monitoring period. In this regard, the time required to obtain a useful trace is often referred to as "warm-up time". Maintaining good contact of the electrode with the skin is also important in avoiding motion artifacts, which generally comprise disturbances in the trace due to relative movement between the electrode and the skin, such as when the patient moves.
It has been found that the warm-up time and motion artifacts may be minimized by utilizing an electrically conductive coating such as a gel or an electrically conductive adhesive on the surface of the electrode in contact with the skin. In this way, an intimate electrical connection is achieved between the electrode and the skin of the patient through the intermediary of the electrically conductive gel or adhesive substance.
Another important consideration, particularly with respect to longer term monitoring, such as in stress testing, during surgery or during in-patient hospital monitoring, is the recovery of the trace following defibrillation. That is, it sometimes becomes necessary to apply a relatively large externally generated electrical potential of short duration to the patient in an attempt to cause the heart to return to normal activity. The abnormal heart activity to be remedied by this procedure is referred to as fibrillation, and hence this application of electrical potential is often referred to as defibrillation. Since the electrocardiograph device measures electrical potential, such sudden applications of a large electrical potential will cause a considerable disturbance in the electrocardiogram trace. It has been found that when the monitoring electrode is in direct contact with the skin of the patient, the electrical charges built up during defibrillation tend to rapidly discharge between the skin and electrode, such that "recovery" of the electrocardiogram trace is relatively rapid.
However, it will be seen that the use of an electrically conductive gel or adhesive prevents the direct skin-to-electrode contact of the type conducive to rapid recovery from defibrillation. In response to these competing considerations, the prior art has developed a relatively complex and expensive silver/silver chloride electrode system. This system has been developed in an effort to accommodate both of the foregoing considerations. Typically, such a system utilizes a sensing element or portion of the electrode which has a silver chloride surface that interfaces with an electrically conductive chloride gel interposed between the sensing element surface and the skin of the patient. The defibrillation recovery is then enhanced by a silver/silver chloride chemical reaction involving the silver chloride on the sensing element and the chloride of the gel. This reaction tends to dissipate the electrical charge relatively rapidly. However, the manufacture of such electrodes and gels and the further manufacture or assembly thereof into prepackaged, disposable electrodes, utilizing this silver/silver chloride technology, is relatively difficult and expensive.